The invention relates to a device for measuring the distance to an object illuminated by a light beam in accordance with the preamble of Patent claim 1.
Different methods are used in monitoring systems for motor vehicles for measuring the distances to obstacles or to vehicles traveling ahead. These methods use for a measuring medium radar impulses, microwaves, ultrasound waves, or infrared radiation. As measuring methods for determining the distance, one can use either the measuring of the transit time or one may use a triangulation method.
Thus, an apparatus is known, for example from German Patent Publication DE 197 04 220 A1 for determining the spacing of an obstacle from a vehicle in which a semiconductor laser, a radar transmitter, or an ultrasound generator is used for the emission of scanning impulses. The transmitted signal that has been reflected by an obstacle is detected by a receiver apparatus. The distance to the obstacle is determined with a time measuring unit for acquiring the time between the sending of the transmitted signal and its detection point of time.
Furthermore, a radar apparatus for a vehicle safety spacing control system is known from German Patent Publication DE 197 13 826 A1 which performs a two dimensional scanning operation by using a rotating polygonal mirror in order to determine, for example relative data such as a distance, a direction, or a speed of a vehicle traveling ahead. In this apparatus an infrared impulse beam is generated by a laser diode. This beam is reflected by a reflection mirror onto the rotating polygonal mirror. The polygonal mirror comprises a plurality of mirror surfaces that are inclined at different angles, so that thereby a determined measuring range is scanned by the transmitter beam in a line-by-line fashion due to the differently inclined mirror surfaces of the polygonal mirror. In case an object is present in this measuring range, the transmitter beam reflected there is detected by a detector element. The two-dimensional scanning operation takes place due to the fact that a pulse beam is directed onto each mirror surface of the polygonal mirror. Thereby, the direction of each beam reflected by each mirror surface is changed by the rotation of the polygonal mirror in such a way that a horizontal scanning operation is accomplished in the horizontal level. The scanning rate of a line can be increased by raising the impulse frequency. When the pulse beam is reflected by a next mirror surface, the horizontal scanning operation is repeated in a different elevational position which is proportional to the angle of inclination of each mirror surface. This scanning operation is repeated at different locations of the elevational direction, so that thereby a two-dimensional scanning operation is realized. The number of different elevational directions which thereby occur, that is the number of the lines, depends on the number of the mirror surfaces which are provided with different inclinations relative to the rotation axis of the polygonal mirror.
If such a measuring apparatus is used in a vehicle, it is possible to adapt the scanning operation for the recognition of a vehicle traveling ahead, to the incline characteristic of the road. For example, if the road rises ahead of the vehicle, the position of a vehicle traveling ahead will also be higher than the position of the vehicle equipped with the measuring apparatus. In order to detect with certainty the transmitter signal reflected by the vehicle traveling ahead also in this situation the number of measurements made in the respective elevational direction is increased, i.e. the impulse frequence of the transmitter signal is increased. In order to be able to perform such a control of the impulse frequency of the transmitter signal, an inclination angle sensor installed in the vehicle provides a respective measured value.
On the one hand the timed control which is expensive to realize is a disadvantage in this known measuring apparatus. The timed control requires that the transmitter must produce an impulse beam with the correct impulse frequency in dependency on the rotation angular position of the polygonal mirror. On the other hand a transmitter is required that is capable of realizing the respective impulse frequencies.
Thus, it is the object of the present invention that the above mentioned apparatus for measuring the distance is improved with regard to a simpler evaluation of the detected light beams.